Regular coded bin reduction for coefficient coding

ABSTRACT

A video decoder can be configured to determine a size of a transform unit, wherein the transform unit comprises N coefficient groups and each coefficient group comprises M coefficients, wherein M and N are integer values; determine a regular bin count threshold for the transform unit based on the size of the transform unit; context decode syntax elements for the transform unit until the first number of regular coded bins is reached; in response to reaching the first number of regular coded bins, bypass decode additional syntax elements of the transform unit; determine values for a first set of coefficients of the transform unit based on the context decoded syntax elements; and determine values for a second set of coefficients of the transform unit based on the additional syntax elements.

This Application claims the benefit of:

U.S. Provisional Patent Application 62/805,625, filed 14 Feb. 2019;

U.S. Provisional Patent Application 62/823,537, filed 25 Mar. 2019; theentire content of each being hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), the High Efficiency Video Coding (HEVC) standard, ITU-TH.265/High Efficiency Video Coding (HEVC), and extensions of suchstandards. The video devices may transmit, receive, encode, decode,and/or store digital video information more efficiently by implementingsuch video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

Video coding (e.g., video encoding and/or video decoding) typicallyinvolves predicting a block of video data from either an already codedblock of video data in the same picture (e.g., intra prediction) or analready coded block of video data in a different picture (e.g., interprediction). In some instances, the video encoder also calculatesresidual data by comparing the predictive block to the original block.Thus, the residual data represents a difference between the predictiveblock and the original block of video data. To reduce the number of bitsneeded to signal the residual data, the video encoder transforms theresidual data into transform coefficients, quantizes the transformcoefficients, and signals the transformed and quantized coefficients inthe encoded bitstream. The compression achieved by the transform andquantization processes may be lossy, meaning that transform andquantization processes may introduce distortion into the decoded videodata. This disclosure describes techniques related to transformcoefficient coding.

In one example, a method for decoding video data includes determining asize of a transform unit, wherein the transform unit comprises Ncoefficient groups and each coefficient group comprises M coefficients,wherein M and N are integer values; determining a regular bin countthreshold for the transform unit based on the size of the transformunit; context decoding syntax elements for the transform unit until thefirst number of regular coded bins is reached, wherein: the contextdecoded syntax elements comprise one or more significance flags, one ormore parity level flags, and one or more first flags, each of the one ormore significance flags indicate if an absolute level for acorresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, each of the one or more first flagsindicates if a corresponding coefficient has an absolute level that isgreater than 2; in response to reaching the first number of regularcoded bins, bypass decoding additional syntax elements of the transformunit; determining values for a first set of coefficients of thetransform unit based on the context decoded syntax elements; anddetermining values for a second set of coefficients of the transformunit based on the additional syntax elements.

In another example, a device for decoding video data includes a memoryconfigured to store video data and one or more processors implemented incircuitry and configured to determine a size of a transform unit of thevideo data, wherein the transform unit comprises N coefficient groupsand each coefficient group comprises M coefficients, wherein M and N areinteger values; determine a regular bin count threshold for thetransform unit based on the size of the transform unit; context decodesyntax elements for the transform unit until the first number of regularcoded bins is reached, wherein: the context decoded syntax elementscomprise one or more significance flags, one or more parity level flags,and one or more first flags, each of the one or more significance flagsindicate if an absolute level for a corresponding coefficient is equalto zero, each of the one or more parity level flags indicates if acorresponding coefficient has an absolute level that is even or odd, andeach of the one or more first flags indicates if a correspondingcoefficient has an absolute level that is greater than 2; in response toreaching the first number of regular coded bins, bypass decodeadditional syntax elements; determine values for a first set ofcoefficients of the transform unit based on the context decoded syntaxelements; and determine values for a second set of coefficients of thetransform unit based on the additional syntax elements.

In another example, a computer readable medium stores instructions thatwhen executed by one or more processors cause the one or more processorsto determine a size of a transform unit of the video data, wherein thetransform unit comprises N coefficient groups and each coefficient groupcomprises M coefficients, wherein M and N are integer values; determinea regular bin count threshold for the transform unit based on the sizeof the transform unit; context decode syntax elements for the transformunit until the first number of regular coded bins is reached, wherein:the context decoded syntax elements comprise one or more significanceflags, one or more parity level flags, and one or more first flags, eachof the one or more significance flags indicate if an absolute level fora corresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, and each of the one or more firstflags indicates if a corresponding coefficient has an absolute levelthat is greater than 2; in response to reaching the first number ofregular coded bins, bypass decode additional syntax elements; determinevalues for a first set of coefficients of the transform unit based onthe context decoded syntax elements; and determine values for a secondset of coefficients of the transform unit based on the additional syntaxelements.

In another example, an apparatus comprises means for determining a sizeof a transform unit, wherein the transform unit comprises N coefficientgroups and each coefficient group comprises M coefficients, wherein Mand N are integer values; means for determining a regular bin countthreshold for the transform unit based on the size of the transformunit; means for context decoding syntax elements for the transform unituntil the first number of regular coded bins is reached, wherein: thecontext decoded syntax elements comprise one or more significance flags,one or more parity level flags, and one or more first flags, each of theone or more significance flags indicate if an absolute level for acorresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, each of the one or more first flagsindicates if a corresponding coefficient has an absolute level that isgreater than 2; means for bypass decoding additional syntax elements ofthe transform unit in response to reaching the first number of regularcoded bins; means for determining values for a first set of coefficientsof the transform unit based on the context decoded syntax elements; andmeans for determining values for a second set of coefficients of thetransform unit based on the additional syntax elements.

In another example, a method for encoding video data includesdetermining a size of a transform unit, wherein the transform unitcomprises N coefficient groups and each coefficient group comprises Mcoefficients, wherein M and N are integer values; determining a regularbin count threshold for the transform unit based on the size of thetransform unit; context encoding syntax elements for the transform unituntil the first number of regular coded bins is reached, wherein: thecontext encoded syntax elements comprises one or more significanceflags, one or more parity level flags, and one or more first flags, eachof the one or more significance flags indicate if an absolute level fora corresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, each of the one or more first flagsindicates if a corresponding coefficient has an absolute level that isgreater than 2, and the context encoded syntax elements represent valuesfor a first set of coefficients of the transform unit; and in responseto reaching the first number of regular coded bins, bypass encodingadditional syntax elements, wherein the additional syntax elementsrepresent values for a second set of coefficients of the transform unit.

In another example, a device for encoding video data includes a memoryconfigured to store video data and one or more processors implemented incircuitry and configured to determine a size of a transform unit,wherein the transform unit comprises N coefficient groups and eachcoefficient group comprises M coefficients, wherein M and N are integervalues; determine a regular bin count threshold for the transform unitbased on the size of the transform unit; context encode syntax elementsfor the transform unit until the first number of regular coded bins isreached, wherein: the context encoded syntax elements comprises one ormore significance flags, one or more parity level flags, and one or morefirst flags, each of the one or more significance flags indicate if anabsolute level for a corresponding coefficient is equal to zero, each ofthe one or more parity level flags indicates if a correspondingcoefficient has an absolute level that is even or odd, each of the oneor more first flags indicates if a corresponding coefficient has anabsolute level that is greater than 2, and the context encoded syntaxelements represent values for a first set of coefficients of thetransform unit; and in response to reaching the first number of regularcoded bins, bypass encode additional syntax elements of the transformunit, wherein the additional syntax elements represent values for asecond set of coefficients of the transform unit.

In another example, a computer readable storage medium storesinstructions that when executed by one or more processors cause the oneor more processors to determine a size of a transform unit of the videodata, wherein the transform unit comprises N coefficient groups and eachcoefficient group comprises M coefficients, wherein M and N are integervalues; determine a regular bin count threshold for the transform unitbased on the size of the transform unit; context decode syntax elementsfor the transform unit until the first number of regular coded bins isreached, wherein: the context decoded syntax elements comprise one ormore significance flags, one or more parity level flags, and one or morefirst flags, each of the one or more significance flags indicate if anabsolute level for a corresponding coefficient is equal to zero, each ofthe one or more parity level flags indicates if a correspondingcoefficient has an absolute level that is even or odd, and each of theone or more first flags indicates if a corresponding coefficient has anabsolute level that is greater than 2; in response to reaching the firstnumber of regular coded bins, bypass decode additional syntax elements;determine values for a first set of coefficients of the transform unitbased on the context decoded syntax elements; and determine values for asecond set of coefficients of the transform unit based on the additionalsyntax elements.

In another example, an apparatus comprises means for determining a sizeof a transform unit, wherein the transform unit comprises N coefficientgroups and each coefficient group comprises M coefficients, wherein Mand N are integer values; means for determining a regular bin countthreshold for the transform unit based on the size of the transformunit; means for context encoding syntax elements for the transform unituntil the first number of regular coded bins is reached, wherein: thecontext encoded syntax elements comprises one or more significanceflags, one or more parity level flags, and one or more first flags, eachof the one or more significance flags indicate if an absolute level fora corresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, each of the one or more first flagsindicates if a corresponding coefficient has an absolute level that isgreater than 2, and the context encoded syntax elements represent valuesfor a first set of coefficients of the transform unit; and means forbypass encoding additional syntax elements in response to reaching thefirst number of regular coded bins, wherein the additional syntaxelements represent values for a second set of coefficients of thetransform unit.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 shows an example scan order for syntax elements.

FIG. 4 shows an illustration of an example template used for selectingprobability models.

FIG. 5 shows an example scan order for syntax elements.

FIG. 6 shows an example illustrating how a Rice parameter may be used toderive a threshold for switching between bypass coded Golomb-Rice codingand Exponential-Golomb coding.

FIG. 7 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 8 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIGS. 9A and 9B are conceptual diagrams illustrating a range updateprocess in binary arithmetic coding.

FIG. 10 is a conceptual diagram illustrating an output process in binaryarithmetic coding.

FIG. 11 is a block diagram illustrating a context adaptive binaryarithmetic coding (CABAC) coder in a video encoder.

FIG. 12 is a block diagram illustrating a CABAC coder in a videodecoder.

FIG. 13 is a flowchart illustrating an example operation of a videoencoder.

FIG. 14 is a flowchart illustrating an example operation of a videodecoder.

FIG. 15 is a flowchart illustrating an example operation of a videoencoder.

FIG. 16 is a flowchart illustrating an example operation of a videodecoder.

DETAILED DESCRIPTION

Video coding (e.g., video encoding and/or video decoding) typicallyinvolves predicting a block of video data from either an already codedblock of video data in the same picture (e.g., intra prediction) or analready coded block of video data in a different picture (e.g., interprediction). In some instances, the video encoder also calculatesresidual data by comparing the predictive block to the original block.Thus, the residual data represents a difference between the predictiveblock and the original block of video data. To reduce the number of bitsneeded to signal the residual data, the video encoder transforms andquantizes the residual data and signals the transformed and quantizedresidual data in the encoded bitstream. The compression achieved by thetransform and quantization processes may be lossy, meaning thattransform and quantization processes may introduce distortion into thedecoded video data.

A video decoder decodes and adds the residual data to the predictiveblock to produce a reconstructed video block that matches the originalvideo block more closely than the predictive block alone. Due to theloss introduced by the transforming and quantizing of the residual data,the reconstructed block may have distortion or artifacts. One commontype of artifact or distortion is referred to as blockiness, where theboundaries of the blocks used to code the video data are visible.

To further improve the quality of decoded video, a video decoder canperform one or more filtering operations on the reconstructed videoblocks. Examples of these filtering operations include deblockingfiltering, sample adaptive offset (SAO) filtering, and adaptive loopfiltering (ALF). Parameters for these filtering operations may either bedetermined by a video encoder and explicitly signaled in the encodedvideo bitstream or may be implicitly determined by a video decoderwithout needing the parameters to be explicitly signaled in the encodedvideo bitstream.

As introduced above, a video encoder transforms residual data to producetransform coefficients. Those transform coefficients may additionally bequantized. In this disclosure, the term transform coefficient, orcoefficient, may refer to either a quantized transform coefficient or anunquantized transform coefficient. This disclosure describes techniquesfor signaling the values of transform coefficients, e.g., quantizedtransform coefficients, from a video encoder to a video decoder. Morespecifically, this disclosure describes techniques related to an entropydecoding process that converts a binary representation of bits to aseries of non-binary valued quantized transform coefficients. Thecorresponding entropy encoding process, which is generally the reverseprocess of entropy decoding, is also described in this disclosure.

This disclosure describes techniques related to transform coefficientcoding. Aspects of transform coefficient coding are described inJVET-K0072, H. Schwarz, T. Nguyen, D. Marpe, T. Wiegand, “Non-CE7:Alternative entropy coding for dependent quantization,” JVET documentJVET-K0072, Ljubljana, Slovenia, July 2018. The techniques describedherein may improve upon the efficiency in encoding and decodingquantized coefficients when compared to the techniques of JVET-K0072.

This disclosure describes techniques for coding coefficients associatedwith residual data in a video coding process. In one example, thisdisclosure describes techniques for determining which syntax elementsare context coded and which syntax elements are bypass coded. In anotherexample, this disclosure describes techniques for determining a Riceparameter used to define codes, e.g., Golomb-Rice codes orExponential-Golomb codes, for coding remaining absolute values ofcoefficient levels for a block of coefficients where context adaptivebinary arithmetic coding (CABAC) is used to code other indications ofsignificant coefficients, such as coefficient levels greater than 1 andcoefficient levels greater than 2. The coefficient levels may be levelsof transform coefficients, in the case of lossy coding, or levels ofcoefficients for which a transform is not applied (i.e., residual pixelvalues), in the case of lossless coding or lossy coding in transformskip mode.

The Rice parameter is a tunable value used to select a codeword set fromthe family of Golomb codes, e.g., Golomb-Rice codes orExponential-Golomb codes. The codes defined by the Rice parameter may beused to code a remaining absolute value of a coefficient level for atleast one coefficient in a transform unit (TU) or a coefficient group(CG), i.e., a block of coefficients. Each of the CGs may be a 4×4transform block or a 4×4 subblock of a transform block of video data.The CGs may include transform coefficients, in the case of lossy coding,or coefficients for which a transform is not applied, in the case oflossless coding or lossy coding in transform skip mode.

The techniques of this disclosure may be applied to any of the existingvideo codecs, such as High Efficiency Video Coding (HEVC), or may beproposed as a promising coding tool for new video coding standards, suchas the Versatile Video Coding (VVC) currently being develop or for otherfuture video coding standards.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch as smartphones, televisions, cameras, display devices, digitalmedia players, video gaming consoles, video streaming device, or thelike. In some cases, source device 102 and destination device 116 may beequipped for wireless communication, and thus may be referred to aswireless communication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for coefficientcoding described herein. Thus, source device 102 represents an exampleof a video encoding device, while destination device 116 represents anexample of a video decoding device. In other examples, a source deviceand a destination device may include other components or arrangements.For example, source device 102 may receive video data from an externalvideo source, such as an external camera. Likewise, destination device116 may interface with an external display device, rather than includean integrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform the techniquesfor coefficient coding described herein. Source device 102 anddestination device 116 are merely examples of such coding devices inwhich source device 102 generates coded video data for transmission todestination device 116. This disclosure refers to a “coding” device as adevice that performs coding (encoding and/or decoding) of data. Thus,video encoder 200 and video decoder 300 represent examples of codingdevices, in particular, a video encoder and a video decoder,respectively. In some examples, source device 102 and destination device116 may operate in a substantially symmetrical manner such that each ofsource device 102 and destination device 116 includes video encoding anddecoding components. Hence, system 100 may support one-way or two-wayvideo transmission between source device 102 and destination device 116,e.g., for video streaming, video playback, video broadcasting, or videotelephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although memory 106 and memory 120 are shown separatelyfrom video encoder 200 and video decoder 300 in this example, it shouldbe understood that video encoder 200 and video decoder 300 may alsoinclude internal memories for functionally similar or equivalentpurposes. Furthermore, memories 106, 120 may store encoded video data,e.g., output from video encoder 200 and input to video decoder 300. Insome examples, portions of memories 106, 120 may be allocated as one ormore video buffers, e.g., to store raw, decoded, and/or encoded videodata.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. File server 114and input interface 122 may be configured to operate according to astreaming transmission protocol, a download transmission protocol, or acombination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., a communicationmedium, storage device 112, file server 114, or the like). The encodedvideo bitstream may include signaling information defined by videoencoder 200, which is also used by video decoder 300, such as syntaxelements having values that describe characteristics and/or processingof video blocks or other coded units (e.g., slices, pictures, groups ofpictures, sequences, or the like). Display device 118 displays decodedpictures of the decoded video data to a user. Display device 118 mayrepresent any of a variety of display devices such as a cathode ray tube(CRT), a liquid crystal display (LCD), a plasma display, an organiclight emitting diode (OLED) display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 6),” Joint Video Experts Team (JVET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting:Gothenburg, SE, 3-12 Jul. 2019, JVET-O2001-vE (hereinafter “VVC Draft6”). The techniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to JEM or VVC. According to JEM or VVC,a video coder (such as video encoder 200) partitions a picture into aplurality of coding tree units (CTUs). Video encoder 200 may partition aCTU according to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of JEM and VVC also provide an affine motion compensationmode, which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofJEM and VVC provide sixty-seven intra-prediction modes, includingvarious directional modes, as well as planar mode and DC mode. Ingeneral, video encoder 200 selects an intra-prediction mode thatdescribes neighboring samples to a current block (e.g., a block of a CU)from which to predict samples of the current block. Such samples maygenerally be above, above and to the left, or to the left of the currentblock in the same picture as the current block, assuming video encoder200 codes CTUs and CUs in raster scan order (left to right, top tobottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information of apicture into CTUs, and partitioning of each CTU according to acorresponding partition structure, such as a QTBT structure, to defineCUs of the CTU. The syntax elements may further define prediction andresidual information for blocks (e.g., CUs) of video data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), then the nodes can befurther partitioned by respective binary trees. The binary treesplitting of one node can be iterated until the nodes resulting from thesplit reach the minimum allowed binary tree leaf node size (MinBTSize)or the maximum allowed binary tree depth (MaxBTDepth). The example ofQTBT structure 130 represents such nodes as having dashed lines forbranches. The binary tree leaf node is referred to as a coding unit(CU), which is used for prediction (e.g., intra-picture or inter-pictureprediction) and transform, without any further partitioning. Asdiscussed above, CUs may also be referred to as “video blocks” or“blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs and are further processed according to predictionand transform without further partitioning.

JVET-K0072 described techniques for Trellis coded quantization (TCQ).When implementing the TCQ proposal of JVET-K0072, a video coder uses twoscalar quantizers switchably for quantization/dequantization. The videocoder determines the scalar quantizer to be used on a currenttransform/quantized coefficient based on the parity (i.e., the leastsignificant bit) of the quantized coefficient that precedes the currenttransform/quantized coefficient in the scanning order.

JVET-K0072 also described a coefficient coding scheme coupled with TCQ.In the JVET-K0072 proposal, a video coder performs the context selectionfor decoding a quantized coefficient based on the quantizer used.Specifically, the significance flag (SIG) of a coefficient indicatingthe coefficient is zero or non-zero has three sets of context models,and the set selected for a particular SIG depends on the quantizer usedfor the associated coefficient. Therefore, when starting toencode/decode a value for SIG of a current coefficient, a video coderdetermines the quantizer for the current coefficient, and thus thecontext set for that coefficient's SIG, based on the parity of thecoefficient in the previous scanning position, which is a known value.

The video coder may determine the parity in the previous scanningposition by, for example, fully reconstructing the previous absolutionlevel (absLevel). This technique, however, potentially limitsthroughput. In JVET-K0071, an alternative, namely that the parity ofeach non-zero coefficient is explicitly transmitted in the bitstream anddecoded before moving to the next scanning position, was proposed andadopted in an early draft of VVC. More details of JVET-K0071 areprovided below.

A TU is divided into non-overlapped subblocks, called coding groups(CGs), of which the size is usually 4×4. The decoding process describedhere is for a 4×4 CG but may easily be extended to any other CG sizes.

Video encoder 200 and video decoder 300 may be configured to processsyntax elements in bitstreams. In one example coding scheme, five syntaxelements are used to represent a value for absLevel. In this scheme, thesyntax element “sig_coeff_flag” is equal to 0 if absLevel is 0;otherwise, the flag is equal to 1. The syntax element “par_level_flag”is present in bitstream if sig_coeff_flag is equal to 1. The syntaxelement “par_level_flag” is equal to 0 if absLevel is an odd number andis equal to 1 if absLevel is an even number. The syntax element“rem_abs_gt1_flag” is present in bitstream if sig_coeff_flag is equalto 1. The syntax element “rem_abs_gt1_flag” is equal to 1 if absLevel isgreater than 2; otherwise, this flag is equal to 0. The syntax element“rem_abs_gt2_flag” is present in bitstream if rem_abs_gt1 flag is equalto 1.

The syntax element “rem_abs_gt2_flag” is equal to 1 if absLevel isgreater than 4; otherwise, the flag is equal to 0. The syntax element“abs_remainder” is present in the bitstream if rem_abs_gt2_flag is equalto 1.

Below and in FIGS. 3 and 5, sig_coeff_flag, par_level_flag,rem_abs_gt1_flag, rem_abs_gt2_flag, and abs_remainder are denoted asSIG, Par, Gt1, Gt2, and rem, respectively, for the simplicity ofdescription. When not parsed from the bitstream, video decoder 300 maybe configured to set any of the above syntax elements to a default valueof 0. Given the values of these five syntax elements, video decoder 300can calculate a value for absLevel as shown in equation (1).absLevel=SIG+Par+(Gt1<<1)+(Gt2<<1)+(rem<<1)  (1)All of the five syntax elements are parsed from bitstream in codingscenarios where absLevel is greater than 6.

FIG. 3 shows an example order for the five syntax elements for all theabsLevels values in a CG. The order in FIG. 3 shows the order of thesyntax elements representing absLevels in a CG as in WET-K0072.

The positions in a CG are scanned in up to three passes. In the firstpass (labeled Pass #1 in FIG. 3), the flags SIG, Par, and Gt1 are parsedfor each of coefficients 0 through 15. If the SIG flag for a particularcoefficient is zero, then that coefficient does not have an associatedPar or Gt1 flag. Thus, in the first pass, only non-zero SIG flags arefollowed by corresponding Par and Gt1 flags. After the first pass, apartial absLevel, denoted as absLevel1, for each position can bedetermined as shown in (2).absLevel=SIG+Par+(Gt1<<1)  (2)

If there is at least one non-zero Gt1 in the first pass, then the secondpass (Pass #2 in FIG. 3) is scanned. In the second pass, Gt2 flags forthe coefficients with non-zero Gt1 s are parsed. The bins in the firstand second passes may all be regular coded, meaning the probabilitydistribution of the bin is modeled by a properly selected context modelas described in more detail below. If there is at lease one non-zero Gt2in the second pass, then a third pass (Pass #3 in FIG. 3) is scanned,and rem values of the coefficients with non-zero Gt2 s are parsed. As arem value is not binary, the bins of the binarized version of a remvalue may all be bypass-coded, meaning the bins are assumed to beuniformly distributed and no context selection is needed.

A video coding device, such as source device 102 and video encoder 200or destination device 116 and video decoder 300, can typically processbypass coded bins much faster than regular coded bins because bypasscoded bins can more easily be processed in parallel due to bypass codedbins not having data dependencies related to context selection andbecause arithmetic coding for bypass bins is generally simpler as itonly requires a right shift versus a table look up as required forregular coded bins. Thus, the overall throughput of a video coder can beimproved by reducing the number of regular coded bins and using bypasscoded bins instead.

There are at least two potential problems with the coefficient codingscheme proposed in JVET-K0072. A first potential problem is that thenumber of regular coded bins is increased. Regular coded bins areprocessed much slower than bypass coded bins due to context selectionand interval subdivision calculation. In JVET-K0072, the number ofregular coded bins in a CG is up to 80, compared with 25 in JEM or HEVC(16 for SIG, 8 for gt1, and 1 for gt2). In the average case, the totalnumber of bins representing absLevels used in JVET-K0072 is 40% to 90%higher when compared to JEM, depending on different bitrates. Anotherpotential problem is that regular coded bins with different sets ofcontexts are interleaved. When bins that use the same logic for contextselection are grouped together, fewer speculative calculations areneeded to process multiple bins per cycle. In JVET-K0072, SIG, Par, andGt1, which use different sets of contexts, are interleaved percoefficient (as illustrated in FIG. 3).

Video encoder 200 and video decoder 300 may be configured to performcontext modelling. The context modelling used in JVET-K0072 is alsobriefly introduced here, as it is modified by this disclosure. The termcontext modelling generally refers to the selection of probabilitymodels (a.k.a. contexts) for a bin-to-decode. In JVET-K0072, SIG, Par,Gt1, and Gt2 flags need context modelling, and the selections depend onthe values of absLevel1s in a local neighbourhood, denoted as N. In someimplementations, positions inside the template but outside the currentTU may be excluded from N.

FIG. 4 shows an illustration of a template that can used for selectingprobability models. Block 150 represents a transform unit. Position X inblock 150 represents a current scan position, and positions Y in block150 represent the local neighbourhood used for determining a probabilitymodel for the current scan position.

For the current scan position (see the black square with the X in FIG.4), video encoder 200 and video decoder 300 may determine the contextindices for the SIG, Par, Gt1, and Gt2 flags, denoted as ctxIdxSig,ctxIdxPar, ctxIdxGt1, and ctxIdxGt2, based on the Y positions. Todetermining ctxIdxSig, ctxIdxPar, ctxIdxGt1, and ctxIdxGt2, videoencoder 200 and video decoder 300 first determine three variables,numSig, sumAbs1, and d. The term numSig represents the number ofnon-zero SIGs in N, which is expressed by (3) below.sumSig=Σ_(i∈N)SIG(i)  (3)

The term sumAbs1 represents the sum of absLevel1 in N, which isexpressed by (4) below.sumAbs1=Σ_(i∈N) absLevel1(i)  (4)

The term d represents the diagonal measure of the current positioninside TU, as expressed by (5) below,d=x+y  (5)

where x and y represent the coordinates of the current position insideTU.

Given sumAbs1 and d, video encoder 200 and video decoder 300 candetermine the context index for decoding SIG as follows:

-   -   For luma, video encoder 200 and video decoder 300 determine        ctxIdxSig as follows:        ctxIdxSig=18*max(0,state−1)+min(sumAbs1,5)+(d<2?12:(d<5?6:0))  (6)    -   For chroma, video encoder 200 and video decoder 300 determine        ctxIdxSig as follows:        ctxIdxSig=12*max(0,state-1)+min(sumAbs1,5)+(d<2?6:0))  (7)

In equations (6) and (7) above, the variable “state” represents thecurrent state of the state machine as defined in JVET-K0072.

Given sumSig, sumAbs1, and d, video encoder 200 and video decoder 300can determine the context index for decoding Par follows:

-   -   If the current scan position is equal to the position of the        last non-zero coefficient, ctxIdxPar is 0.    -   Otherwise,        -   For luma, ctxIdxPar is determined by:            ctxIdxPar=1+min(sumAbs1−numSig,4)+(d==0?15:(d<3?10:(d<10?5:0)))  (8)    -   For chroma, ctxIdxPar is determined by (9)        ctxIdxPar=1+min(sumAbs1−numSig,4)+(d==0?5:0)  (9)        ctxIdxGt1 and ctxIdxGt2 are set the value of ctxIdxPar.

Video encoder 200 and video decoder 300 may perform absLevelreconstruction is the same manner as described in JVET-K0072.

According to a technique of this disclosure, in order to reduce thenumber of regular coded bins, video encoder 200 and video decoder 300may use a predetermined number (C1) of regular coded bins in acoefficient group to code the sig-par-gt1 pass (i.e. first pass) ofcoefficients in a coefficient group until C1 regular coded bin thresholdis reached. The techniques of this disclosure will be described withrespect to 4×4 CGs, but the techniques of this disclosure are generallyapplicable to CGs of any size.

In one example implementation, video encoder 200 and video decoder 300are configured to code all bins of the last coefficient where theregular bin count threshold C1 is reached as regular bins. After thefirst pass, which may include only regular coded bins, video encoder 200and video decoder 300 may code a predetermined number (C2) of gt2 flags(if available) using regular bins. C2 with value equal to 2 is oneparticular choice that gives good performance-complexity trade off;however, other values of C2 may also be used. After the second pass, allremaining levels of coefficients after the first and second pass may becoded using bypass bins. Specifically, video encoder 200 and videodecoder 300 may code remaining levels using the Rice/Golomb codingcombination described below following the coding scan order startingwith the first coefficient with remaining level info as shown in FIG. 5.For coding of Gt2 flags, the last regular coded Gt2 flag cannot exceedthe location of the last regular coded Gt1 flag in first pass, i.e. inFIG. 5, j<=K, even C2 if C2 threshold is not reached.

FIG. 5 shows an example scan order for syntax elements. In the exampleof FIG. 5, in a first pass, video encoder 200 and video decoder 300 codeSIG, Par, and Gt1 flags until a threshold number of regular coded binsis reached. In the example of FIG. 5, the threshold number of regularcoded bins is represented as C1 and occurs at coefficient K. Once thethreshold number of regular coded bins is reached, then video encoder200 and video decoder 300 may code the remaining flags for coefficientK, such as Par and Gt1 if present for coefficient K, using regular codedbins and then the first pass is complete. In a second pass, videoencoder 200 and video decoder 300 code Gt2 flags until a secondthreshold number of regular coded bins is reached. In the example ofFIG. 5, the second threshold number of regular coded bins is representedas C2 and occurs at coefficient j. Depending on the particular codingscenario, coefficient j may be any of coefficients 1 through N. Aftercoding the Gt2 flag for coefficient j, the second pass is complete.

For the third pass, video encoder 200 and video decoder 300 may performremaining level coding for those coefficients that have a remaininglevel to code. For coefficients K+1 through N, the remaining levelrepresents the absolute level of the coefficient. For coefficients 0through K, the remaining level represents the difference between anintermediate value and the absolute level. The intermediate level for acoefficient is a level determined based on the SIG, Par, and Gt1 flags,and for some coefficients, also based on the Gt2 flag. Video encoder 200and video decoder 300 may code the remLevel part of the coefficients,whose prefix bins are coded as regular bins in the first and secondpasses, using the scheme described in JVET-K0072 where a Rice Parameter(ricePar) is determined following the following rule:

-   -   If sumAbs—numSig is less than 12, ricePar is set equal to 0;    -   Otherwise, if sumAbs—numSig is less than 25, ricePar is set        equal to 1;    -   Otherwise, ricePar is set equal to 2.

The ricePar is used to derive the threshold to switch between bypasscoded Golomb-Rice code and the Exponential-Golomb code JVET-K0072 asillustrated in FIG. 6.

This disclosure describes techniques for coding remLevel values forcoefficients that are not regular bin coded in the first pass, that is,the remaining coefficients of a CG after reaching the C1 regular codedbin threshold. This disclosure describes techniques for a modifiedricePar derivation that depends on the sum of absolute values ofneighboring coefficients. The neighboring coefficients may, for example,be the same neighboring coefficients identified in FIG. 4 or may bedifferent coefficients.

In some examples, video encoder 200 and video decoder 300 may perform amapping of coefficient levels to codewords to be coded depending on aTCQ state (assumed to be 0 for non-TCQ cases) and the sum of absolutevalues in the neighborhood template. In addition to the sum of absolutevalue and the TCQ state dependency, dependencies on the coefficientposition such as the diagonal position within a TU may also be used asparameters to define the threshold value (pos0) used for remapping ofthe coefficient values to codewords.

Video encoder 200 and video decoder 300 may be configured to derive aricePar as follows. Video encoder 200 and video decoder 300 calculate asum of absolute values (i.e., absolute coefficient values) of thecoefficients in the context template

${sumAbsLevel} = {\sum\limits_{i \in N}{{absLevel}(i)}}$

Video encoder 200 and video decoder 300 limit the sumAbsLevel to amaximum of 31 for riceParameter look up from the below table. Videoencoder 200 and video decoder 300 determine a value for r based on thelesser of sumAbsLevel or 31. The table below shows an array of 32 valuesat positions 0 to 31. The value of r is equal to the value at positionsumAbsLevel or position 31 if sumAbsLevel is greater than 31.

r=g_auGoRiceParsCoeff[min(sumAbsLevel, 31)], where const uint32tg_auiGoRiceParsCoeff[32]={0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 2,2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3};

Video encoder 200 and video decoder 300 derive a pos0 value to be usedfor coding of the remaining level as follows:

pos0=g_auigoRicePosCoeff0[max(0, state−1)][sumAbsLevel], where constuint32t_g_auiGoRicePosCoeff0[3][32]={{0, 0, 0, 0, 0, 1, 2, 2, 2, 2, 3,3, 4, 4, 4, 4, 4, 4, 5, 5, 6, 7, 7, 8, 8, 8, 8, 8, 8, 9, 9, 9}, {1, 1,1, 1, 2, 3, 4, 4, 5, 6, 6, 7, 8, 8, 8, 9, 10, 11, 12, 12, 12, 12, 14,15, 15, 15, 16, 16, 16, 16, 17, 17}, {1, 1, 2, 2, 2, 3, 4, 4, 5, 6, 6,7, 8, 8, 9, 9, 10, 11, 12, 12, 12, 12, 14, 15, 15, 16, 16, 16, 16, 17,17, 17}};

Video encoder 200 and video decoder 300 computes the absLevel0 value tobe coded using rice/Exp Golomb coding (with rice Parameter value rcomputed as described above) for a coefficient with absLevel value asfollows:

- absLevel0 is defined as if (absLevel == 0) { absLevel0 = pos0; } elseif (absLevel <= pos0) { absLevel0 = absLevel − 1; } else { absLevel0 =absLevel; }

Using this technique, video encoder 200 and video decoder 300 may groupregular coded bins and bypass coded bins in a coefficient group whichincreases throughput for CABAC parsing.

According to another technique of this disclosure, the parity bit thatdrives the state machine may not change the state after a certain numberof coefficients are coded in a coefficient group. In this case, there isno need to interleave the significance and the parity bins because thestate does not need to change for coding efficiency. So, after a certainnumber of coefficients are decoded in a coefficient group, thecoefficient coding scheme can take the form of sig-gt1-gt2-remLevelordered coding where dependency on the state of TCQ is removed that canbe moved to bypass coded bins (reducing regular coded bins) at finergranularity where sig, greater than 1 (gt1), greater than 2 (gt2)regular coded bins can be converted to bypass coded bins.

Video encoder 200 and video decoder 300 may be configured to implementTU-level regular bin counting. In VVC and HEVC, the number of regularbin count restriction is imposed currently at the coefficient group (CG)level. Coefficient groups are in units of 16 coefficients which are(4×4), 2×8, 8×2, 1×16,16×1 or sometimes at 2×2 level (for chroma).Currently in VVC, 32 regular coded bins are allowed for 16 coefficientCGs. For 2×2 chroma CGs, 8 regular coded bins are allowed. Basically, itis limited at 2 regular bins per coefficient at CG level. Thisrestriction addresses the worst-case limit for regular bin count forcoefficient count by imposing the restrictions at CG level. Therestriction can be relaxed by imposing a limit at TU level (TUs arecomposed of group of CGs for coefficient coding) which would typicallybe proportional to its size. For example, an 8×8 TU would have four 4×4CGs. The regular coded bin count for TU can be set to total of 4×32=128for luma component. Relaxing the CG level limit to TU level wouldincrease the average number of regular coded bins.

Starting from last position, video encoder 200 and video decoder 300 maybe configured to code a predetermined number of regular coded bins forsig, gt1, par, gt3 (abs_level greater than 3 flag) flags (i.e. 4 flags).Video encoder 200 and video decoder 300 may be configured to code theseflags in groups of 4 (if present) until the regular coded bin count isreached. So, in this proposed scheme it would be possible to code16*4=64 regular coded bins per CG (rather than 32 for current case inVVC) until target limit for total number of regular coded bins isreached. After reaching the regular coded bin count, video encoder 200and video decoder 300 may represent the rest of the coefficients fullyby bypass coding as dec_abs_level described in VVC draft 4 and as alsodescribed above with respect to remaining level coding.

The limit for regular bin count may be set by expanding current limit atCG level. So, a TU with N 16—coefficient CGs may have total bit regularbin count limit of N×32, basically 2 regular bin per coefficient. Thiscan be expanded to other limits, i.e. some regular bin count percoefficient target. In addition, depending on the last past coefficientposition, the target can be set according to number of coefficientsgroups or number of coefficients covered during the scanning of codedcoefficients. The target can be set by regular bin count per coefficientfor the number of coefficients in the scan covering the codedcoefficients.

The limit can also discount the not coded coefficient groups for settingthe target limit or not considering the zero out regions of some typesof TU s. Thus, if it is known that no coefficients are present in a CGin advance, then video encoder 200 and video decoder 300 may set a lowertotal TU level to reduce the average number regular coded bins.

With this new design, only the last coded coefficient group coding whereregular bin count is reached would be similar to the current CG levelcoefficient coding where you would have mixture of regular coded bins(sig, gt1, par, gt2), and fully coded coefficient levels in a CG. Thereis always, abs_remainder [3] codes for all coefficients which haveregular coded bin portions for representation of half of the remaininglevel.

The target regular bin per coefficient can be made to depend on TU sizeas well as number of CGs to be coded in a TU. So larger TUs with manycoefficients may be assigned a lower regular bin per coefficient targetto reduce the average regular bin count.

Video encoder 200 and video decoder 300 may limit the setting of theTU-level regular bin count for sig_flag, gt1_flag, parity_flag, gt3_flag(old gt2_flag) in a TU to 28 times the CG count for 16 coefficient CGsand (7 for each of the four coefficient CGs), or 28 for 16 coefficient(7 for each of the 4 coefficient CGs) CGs for Luma and 24 for 16coefficient chroma and 6 for 4 coefficient chroma CGs.

In another examples, instead of being dependent on TU-size or number ofcoded CGs, the TU level limit may be set to a total number (e.g., totalnumber of regularly coded bins) defined by non zeroed-out regions oftransform units (currently large transforms dimension greater than 32get zeroed out to 32×N, N×32 region, N<=32) Some transforms utilizingmultiple transform sets (MTS) utilize zero out at the transform level(dimension greater than 16 get zeroed out to 16×N, N×16 region, N<=16).Also, secondary transforms (NSST) may utilize a similar zero out schemeat a smaller limit. Accordingly, in some examples, the limit (e.g.,limit of regularly coded bins) may be defined by the number of samplesin the non-zero out regions times a factor (e.g., 2, 1.75 bins/persample). A factor of 2 would correspond to 32 regular bins for a 16coefficient CG. A factor of 1.75 would correspond to 28 bin per 16coefficient CG. Different limits can be set for chroma TUs. Thistechnique may be combined with discounting the bins allocated for theCGs that are not coded, so a total starting limit is set at TU level,and as the coefficients are decoded, if a skipped or not coded CG isencountered the limit is reduced by the regular bins allocated for theCG. In some examples, this may only be applied to skipped CGs (CGs thatare zero from zero out property of some transforms).

FIG. 7 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 7 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 video coding standard in development. However,the techniques of this disclosure are not limited to these video codingstandards and are applicable generally to video encoding and decoding.

In the example of FIG. 7, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 7 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that canprogrammed to perform various tasks and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, the one or more of the units maybe distinct circuit blocks (fixed-function or programmable), and in someexamples, the one or more units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theobject code of the software that video encoder 200 receives andexecutes, or another memory within video encoder 200 (not shown) maystore such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 210 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,uncoded version of the current block from video data memory 230 and theprediction block from mode selection unit 202. Residual generation unit204 calculates sample-by-sample differences between the current blockand the prediction block. The resulting sample-by-sample differencesdefine a residual block for the current block. In some examples,residual generation unit 204 may also determine differences betweensample values in the residual block to generate a residual block usingresidual differential pulse code modulation (RDPCM). In some examples,residual generation unit 204 may be formed using one or more subtractorcircuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder20 and video decoder 30 may also support asymmetric partitioning for PUsizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit does not further partition a CUinto PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 120 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as fewexamples, mode selection unit 202, via respective units associated withthe coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block. Thus,

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the coefficient blocks associated withthe current block by adjusting the QP value associated with the CU.Quantization may introduce loss of information, and thus, quantizedtransform coefficients may have lower precision than the originaltransform coefficients produced by transform processing unit 206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 224 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 224 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200,including the syntax elements described above for coefficient coding.For example, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding blocks andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data including a memory configured to store video data, and one ormore processing units implemented in circuitry and configured todetermine a size of a transform unit, wherein the transform unitcomprises N coefficient groups and each coefficient group comprises Mcoefficients, wherein M and N are integer values; determine a regularbin count threshold for the transform unit based on the size of thetransform unit; context encode syntax elements for the transform unituntil the first number of regular coded bins is reached; and in responseto reaching the first number of regular coded bins, bypass encodeadditional syntax elements of the transform unit, wherein the additionalsyntax elements represent values for a second set of coefficients of thetransform unit.

FIG. 8 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 8 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 is describedaccording to the techniques of JEM and HEVC. However, the techniques ofthis disclosure may be performed by video coding devices that areconfigured to other video coding standards.

In the example of FIG. 8, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Prediction processing unit 304includes motion compensation unit 316 and intra-prediction unit 318.Prediction processing unit 304 may include addition units to performprediction in accordance with other prediction modes. As examples,prediction processing unit 304 may include a palette unit, anintra-block copy unit (which may form part of motion compensation unit318), an affine unit, a linear model (LM) unit, or the like. In otherexamples, video decoder 300 may include more, fewer, or differentfunctional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. CPB memory 320and DPB 314 may be provided by the same memory device or separate memorydevices. In various examples, CPB memory 320 may be on-chip with othercomponents of video decoder 300, or off-chip relative to thosecomponents.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to executed by processing circuitry of video decoder 300.

The various units shown in FIG. 8 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 7, fixed-function circuits referto circuits that provide particular functionality and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, the one ormore of the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, the one or more units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements,including the syntax elements described above for coefficient coding.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 7).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 7).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Asdiscussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures fromDPB for subsequent presentation on a display device, such as displaydevice 118 of FIG. 1.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured todetermine a size of a transform unit of the video data, wherein thetransform unit comprises N coefficient groups and each coefficient groupcomprises M coefficients, wherein M and N are integer values; determinea regular bin count threshold for the transform unit based on the sizeof the transform unit; context decode syntax elements for the transformunit until the first number of regular coded bins is reached; inresponse to reaching the first number of regular coded bins, bypassdecode additional syntax elements; determine values for a first set ofcoefficients of the transform unit based on the context decoded syntaxelements; and determine values for a second set of coefficients of thetransform unit based on the additional syntax elements.

FIGS. 9A and 9B show examples of a CABAC process at a bin n. In example400 of FIG. 9A, at bin n the range at bin 2 includes the RangeMPS andRangeLPS given by the probability of the least probable symbol (LPS)(p_(σ)) given a certain context state (σ). Example 400 shows the updateof the range at bin n+1 when the value of bin n is equal to the mostprobable symbol (MPS). In this example, the low stays the same, but thevalue of the range at bin n+1 is reduced to the value of RangeMPS at binn. Example 402 of FIG. 9B shows the update of the range at bin n+1 whenthe value of bin n is not equal to the MPS (i.e., equal to the LPS). Inthis example, the low is moved to the lower range value of RangeLPS atbin n. In addition, the value of the range at bin n+1 is reduced to thevalue of RangeLPS at bin n.

In one example of the HEVC video coding process, range is expressed with9 bits and the low with 10 bits. There is a renormalization process tomaintain the range and low values at sufficient precision. Therenormalization occurs whenever the range is less than 256. Therefore,the range is always equal or larger than 256 after renormalization.Depending on the values of range and low, the binary arithmetic coder(BAC) outputs to the bitstream, a ‘0,’ or a ‘1,’ or updates an internalvariable (called BO: bits-outstanding) to keep for future outputs. FIG.10 shows examples of BAC output depending on the range. For example, a‘1’ is output to the bitstream when the range and low are above acertain threshold (e.g., 512). A ‘0’ is output to the bitstream when therange and low are below a certain threshold (e.g., 512). Nothing isoutput to the bitstream when the range and lower are between certainthresholds. Instead, the BO value is incremented and the next bin isencoded.

In the CABAC context model of H.264/AVC and in some examples of HEVC,there are 128 states. There are 64 possible LPS probabilities (denotedby state σ) that can be from 0 to 63. Each MPS can be zero or one. Assuch, the 128 states are 64 state probabilities times the 2 possiblevalues for MPS (0 or 1). Therefore, the state can be indexed with 7bits.

To reduce the computation of deriving LPS ranges (rangeLPSa), resultsfor all cases may pre-calculated and stored as approximations in alook-up table. Therefore, the LPS range can be obtained without anymultiplication by using a simple table lookup. Avoiding multiplicationcan be important for some devices or applications, since this operationmay cause significant latency in many hardware architectures.

A 4-column pre-calculated LPS range table may be used instead of themultiplication. The range is divided into four segments. The segmentindex can be derived by the question (range>>6) &3. In effect, thesegment index is derived by shifting and dropping bits from the actualrange. The following Table 1 shows the possible ranges and theircorresponding indexes.

TABLE 1 Range Index Range 256-319 320-383 384-447 448-511 (range >> 6) &3 0 1 2 3

The LPS range table has then 64 entries (one for each probability state)times 4 (one for each range index). Each entry is the Range LPS, thatis, the value of multiplying the range times the LPS probability. Anexample of part of this table is shown in the following Table 2. Table 2depicts probability states 9-12. In one proposal for HEVC, theprobability states may range from 0-63.

TABLE 2 RangeLPS RangeLPS Prob State (σ) Index 0 Index 1 Index 2 Index 3. . . . . . . . . . . . . . .  9 90 110 130 150 10 85 104 123 142 11 81 99 117 135 12 77  94 111 128 . . . . . . . . . . . . . . .

In each segment (i.e., range value), the LPS range of each probabilitystate, is pre-defined. In other words, the LPS range of a probabilitystate, is quantized into four values (i.e., one value for each rangeindex). The specific LPS range used at a given point depends on whichsegment the range belongs to. The number of possible LPS ranges used inthe table is a trade-off between the number of table columns (i.e., thenumber of possible LPS range values) and the LPS range precision.Generally speaking, more columns results in smaller quantization errorsof LPS range values, but also increases the need for more memory tostore the table. Fewer columns increases quantization errors, but alsoreduces the memory needed to store the table.

As described above, each LPS probability state has a correspondingprobability. The probability p for each state is derived as follows:p _(σ) =αp _(σ−1)where the state σ is from 0 to 63. The constant α represents the amountof probability change between each context state. In one example,α=0.9493, or, more precisely, α=(0.01875/0.5)^(1/63). The probability atstate σ=0 is equal to 0.5 (i.e., p₀=½). That is, at context state 0, theLPS and MPS are equally probable. The probability at each successivestate is derived by multiplying the previous state by α. As such, theprobability of the LPS occurring at context state α=1 isp₀*0.9493(0.5*0.9493=0.47465). As such, as the index of state αincreases, the probability of the LPS occurring goes down.

CABAC is adaptive because the probability states are updated in order tofollow the signal statistics (i.e., the values of previously codedbins). The update process is as follows. For a given probability state,the update depends on the state index and the value of the encodedsymbol identified either as an LPS or an MPS. As a result of theupdating process, a new probability state is derived, which consists ofa potentially modified LPS probability estimate and, if necessary, amodified MPS value.

In the event of a bin value equaling the MPS, a given state index may beincremented by 1. This is for all states except when an MPS occurs atstate index 62, where the LPS probability is already at its minimum (orequivalently, the maximum MPS probability is reached). In this case, thestate index 62 remains fixed until an LPS is seen, or the last bin valueis encoded (state 63 is used for the special case of the last binvalue). When an LPS occurs, the state index is changed by decrementingthe state index by a certain amount, as shown in the equation below.This rule applies in general to each occurrence of a LPS with thefollowing exception. Assuming a LPS has been encoded at the state withindex σ=0, which corresponds to the equi-probable case, the state indexremains fixed, but the MPS value will be toggled such that the value ofthe LPS and MPS will be interchanged. In all other cases, no matterwhich symbol has been encoded, the MPS value will not be altered. Thederivation of the transition rules for the LPS probability is based onthe following relation between a given LPS probability p_(old) and itsupdated counterpart p_(new):

-   -   p_(new)=max(α p_(old), p₆₂) if a MPS occurs    -   p_(new)=(1−α)+α p_(old) if a LPS occurs

With regard to a practical implementation of the probability estimationprocess in CABAC, it is important to note that all transition rules maybe realized by at most two tables each having 63 entries of 6-bitunsigned integer values. In some examples, state transitions may bedetermined with a single table TransldxLPS, which determines, for agiven state index σ, the new updated state index TransldxLPS [σ] in casean LPS has been observed. The MPS-driven transitions can be obtained bya simple (saturated) increment of the state index by the fixed value of1, resulting in an updated state index min(σ+1, 62). Table 3 below is anexample of a partial TransldxLPS table.

TABLE 3 TransIdxLPS Prob State (σ) New State TransIdxLPS [σ] . . . . . . 9 6 10 8 11 8 12 8 . . . . . .

The techniques described above with respect to FIGS. 9A, 9B and 10merely represent one example implementation of CABAC. It should beunderstood that the techniques of this disclosure are not limited onlyto this described implementation of CABAC. For example, in older BACapproaches (e.g., the BAC approach used in H.264/AVC), the tablesRangeLPS and TransldxLPS were tuned for low resolution videos, (i.e.,common intermediate format (CIF) and quarter-CIF (QCIF) videos). WithHEVC and future codecs such as VVC, a large amount of video content ishigh definition (HD) and, in some cases, greater than HD. Video contentthat is HD or greater than HD resolution tends to have differentstatistics than the 10-year-old QCIF sequences used to developH.264/AVC. As such, the tables RangeLPS and TransldxLPS from H.264/AVCmay cause adaptation between states in a manner that is too quick. Thatis, the transitions between probability states, especially when an LPSoccurs, can be too great for the smoother, higher resolution content ofHD video. Thus, the probability models used according to conventionaltechniques may not be as accurate for HD and extra-HD content. Inaddition, as HD video content includes a greater range of pixel values,the H.264/AVC tables do not include enough entries to account for themore extreme values that may be present in HD content.

As such, for HEVC and for future coding standards such as VVC, theRangeLPS and TransldxLPS tables may be modified to account for thecharacteristics of this new content. In particular, the BAC processesfor HEVC and future coding standards may use tables that allow for aslower adaptation process and may account for more extreme cases (i.e.,skewed probabilities). Thus, as one example, the RangeLPS andTransldxLPS tables may be modified to achieve these goals by includingmore probability states and ranges than used in BAC with H.264/AVC orHEVC.

FIG. 11 is a block diagram of an example entropy encoding unit 220 thatmay be configured to perform CABAC in accordance with the techniques ofthis disclosure. A syntax element 418 is input into the entropy encodingunit 220. If the syntax element is already a binary-value syntax element(i.e., a syntax element that only has a value of 0 and 1), the step ofbinarization may be skipped. If the syntax element is a non-binaryvalued syntax element (e.g., a syntax element represented by multiplebits, such as transform coefficient levels), the non-binary valuedsyntax element is binarized by binarizer 420. Binarizer 420 performs amapping of the non-binary valued syntax element into a sequence ofbinary decisions. These binary decisions are often called “bins.” Forexample, for transform coefficient levels, the value of the level may bebroken down into successive bins, each bin indicating whether or not theabsolute value of coefficient level is greater than some value. Forexample, bin 0 (sometimes called a significance flag) indicates if theabsolute value of the transform coefficient level is greater than 0 ornot. Bin 1 indicates if the absolute value of the transform coefficientlevel is greater than 1 or not, and so on. A unique mapping may bedeveloped for each non-binary valued syntax element.

Each bin produced by binarizer 420 is fed to the binary arithmeticcoding side of entropy encoding unit 220. That is, for a predeterminedset of non-binary valued syntax elements, each bin type (e.g., bin 0) iscoded before the next bin type (e.g., bin 1). Coding may be performed ineither regular mode or bypass mode. In bypass mode, bypass coding engine426 performs arithmetic coding using a fixed probability model, forexample, using Golomb-Rice or exponential Golomb coding. Bypass mode isgenerally used for more predictable syntax elements.

Coding in regular mode involves performing CABAC. Regular mode CABAC isfor coding bin values where the probability of a value of a bin ispredictable given the values of previously coded bins. The probabilityof a bin being an LPS is determined by context modeler 422. Contextmodeler 422 outputs the bin value and the context model (e.g., theprobability state σ). The context model may be an initial context modelfor a series of bins, or may be determined based on the coded values ofpreviously coded bins. As described above, the context modeler mayupdate the state based on whether or not the previously-coded bin was anMPS or an LPS.

After the context model and probability state σ are determined bycontext modeler 422, regular coding engine 424 performs BAC on the binvalue. According to the techniques of this disclosure, regular codingengine 424 performs BAC using TransldxLPS table 430 that includes morethan 64 probability states G. In one example, the number of probabilitystates is 128. TransldxLPS is used to determine which probability stateis used for a next bin (bin n+1) when the previous bin (bin n) is anLPS. Regular coding engine 424 may also use a RangeLPS table 128 todetermine the range value for an LPS given a particular probabilitystate G. However, according to the techniques of this disclosure, ratherthan using all possible probability states a of the TransldxLPS table430, the probability state indexes a are mapped to grouped indexes foruse in the RangeLPS table. That is, each index into the RangeLPS table428 may represent two or more of the total number of probability states.The mapping of probability state index a to grouped indexes may belinear (e.g., by dividing by two), or may be non-linear (e.g., alogarithmic function or mapping table).

In other examples of the disclosure, the difference between successiveprobability states may be made smaller by setting the parameter α to begreater than 0.9493. In one example, α=0.9689. In another example of thedisclosure, the highest probability (p₀) of an LPS occurring may be setto be lower than 0.5. In one example, p₀ may be equal to 0.493.

In accordance with one or more techniques of this disclosure, as opposedto using the same value of a variable used to update a probability statein a binary arithmetic coding process (e.g., one or more of a windowsize, a scaling factor (α), and a probability updating speed), entropyencoding unit 220 may use different values of the variable for differentcontext models and/or different syntax elements. For instance, entropyencoding unit 220 may determine, for a context model of a plurality ofcontext models, a value of a variable used to update a probability statein a binary arithmetic coding process and update the probability statebased on the determined value.

FIG. 12 is a block diagram of an example entropy decoding unit 302 thatmay be configured to perform CABAC in accordance with the techniques ofthis disclosure. The entropy decoding unit 302 of FIG. 12 performs CABACin an inverse manner as that of entropy encoding unit 220 described inFIG. 11. Coded bits from bitstream 448 are input into entropy decodingunit 302. The coded bits are fed to either context modeler 450 or bypasscoding engine 452 based on whether or not the coded bits were entropycoded using bypass mode or regular mode. If the coded bits were coded inbypass mode, bypass decoding engine 452 may, for example, useGolomb-Rice or exponential Golomb decoding to retrieve the binary-valuedsyntax elements or bins of non-binary syntax elements.

If the coded bits were coded in regular mode, context modeler 450 maydetermine a probability model for the coded bits and regular decodingengine 454 may decode the coded bits to produce bins of non-binaryvalued syntax elements (or the syntax elements themselves ifbinary-valued). After the context model and probability state σ isdetermined by context modeler 450, regular decoding engine 454 performsBAC on the bin value. According to the techniques of this disclosure,regular decoding engine 454 performs BAC using TransldxLPS table 458that includes more than 64 probability states G. In one example, thenumber of probability states is 128, although other numbers ofprobability states could be defined, consistent with the techniques ofthis disclosure. TransldxLPS table 458 is used to determine whichprobability state is used for a next bin (bin n+1) when the previous bin(bin n) is an LPS. Regular decoding engine 454 may also use a RangeLPStable 456 to determine the range value for an LPS given a particularprobability state G. However, according to the techniques of thisdisclosure, rather than using all possible probability states a of theTransldxLPS table 458, the probability state indexes a are mapped togrouped indexes for use in RangeLPS table 456. That is, each index intoRangeLPS table 456 may represent two or more of the total number ofprobability states. The mapping of probability state index a to groupedindexes may be linear (e.g., by dividing by two), or may be non-linear(e.g., a logarithmic function or mapping table).

In other examples of the disclosure, the difference between successiveprobability states may be made smaller by setting the parameter α to begreater than 0.9493. In one example, α=0.9689. In another example of thedisclosure, the highest probability (p₀) of an LPS occurring may be setto be lower than 0.5. In one example, p₀ may be equal to 0.493.

After the bins are decoded by regular decoding engine 454, a reversebinarizer 460 may perform a reverse mapping to convert the bins backinto the values of the non-binary valued syntax elements.

FIG. 13 is a flowchart illustrating an example operation of a videoencoder for encoding a current block of video data. The current blockmay include a current CU. Although described with respect to videoencoder 200 (FIGS. 1 and 7), it should be understood that other devicesmay be configured to perform an operation similar to that of FIG. 13.

In this example, video encoder 200 initially predicts the current block(550). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (552). To calculate the residual block, videoencoder 200 may calculate a difference between the original, uncodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(554). Next, video encoder 200 may scan the quantized transformcoefficients of the residual block (556). During the scan, or followingthe scan, video encoder 200 may entropy encode the coefficients (558).For example, video encoder 200 may encode the coefficients using CAVLCor CABAC. Video encoder 200 may then output the entropy coded data ofthe block (560).

FIG. 14 is a flowchart illustrating an example operation of a videodecoder for decoding a current block of video data. The current blockmay include a current CU. Although described with respect to videodecoder 300 (FIGS. 1 and 8), it should be understood that other devicesmay be configured to perform an operation similar to that of FIG. 14.

Video decoder 300 may receive entropy coded data for the current block,such as entropy coded prediction information and entropy coded data forcoefficients of a residual block corresponding to the current block(570). Video decoder 300 may entropy decode the entropy coded data todetermine prediction information for the current block and to reproducecoefficients of the residual block (572). Video decoder 300 may predictthe current block (574), e.g., using an intra- or inter-prediction modeas indicated by the prediction information for the current block, tocalculate a prediction block for the current block. Video decoder 300may then inverse scan the reproduced coefficients (576), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize and inverse transform the coefficients to produce aresidual block (578). Video decoder 300 may ultimately decode thecurrent block by combining the prediction block and the residual block(580).

FIG. 15 is a flowchart illustrating an example operation of a videoencoder for encoding a current block of video data. The current blockmay include a current CU. Although described with respect to videoencoder 200 (FIGS. 1 and 7), it should be understood that other devicesmay be configured to perform an operation similar to that of FIG. 15.

In the example of FIG. 15, video encoder 200 determines a size of atransform unit (600). The transform unit includes N coefficient groups,and each coefficient group includes M coefficients. M and N are integervalues. In one example, the transform unit may be a luma transform unit,and M may equal 16. In another example, the transform unit may be achroma transform unit, and M may equal 4.

Video encoder 200 determines a regular bin count threshold for thetransform unit based on the size of the transform unit (602). Todetermine the regular bin count threshold for the transform unit basedon the size of the transform unit, video encoder 200 may, for example,set the regular bin count threshold for the transform unit equal toN*28. To determine the regular bin count threshold for the transformunit based on the size of the transform unit, video encoder 200 may, forexample, set the regular bin count threshold for the transform unitequal to 1.75 multiplied by a total number of coefficients in thetransform unit.

In one example, to determine the size of the transform unit, videoencoder 200 may determine a size of a non-zeroed out region of thetransform unit, and to determine the regular bin count threshold for thetransform unit based on the size of the transform unit, video encoder200 may set the regular bin count threshold for the transform unit equalto 28 multiplied by a number of coefficient groups in the non-zeroed outregion of the transform unit. In another example, to determine the sizeof the transform unit, video encoder 200 may determine a size of anon-zeroed out region of the transform unit, and to determine theregular bin count threshold for the transform unit based on the size ofthe transform unit, video encoder 200 may set the regular bin countthreshold for the transform unit equal to 1.75 multiplied by a totalnumber of coefficients in the non-zeroed out region of the transformunit. In an example where the transform unit is a chroma transform unit,to determine the regular bin count threshold for the transform unitbased on the size of the transform unit, video encoder 200 may set theregular bin count threshold for the transform unit equal to N*7.

Video encoder 200 context encodes syntax elements for the transform unituntil the first number of regular coded bins is reached (604). Thecontext encoded syntax elements may, for example, include one or moresignificance flags (e.g., SIG as described above), one or more paritylevel flags (e.g., Par as described above), and one or more first flags(e.g., Gt1 and/or Gt2 as described above).

In response to reaching the first number of regular coded bins, videoencoder 200 bypass encodes additional syntax elements of the transformunit (606). The additional syntax elements represent values for a secondset of coefficients of the transform unit. To bypass encode theadditional syntax elements, video encoder 200 may encode the additionalsyntax elements using one or both of Golomb-Rice coding orExponential-Golomb coding.

Video encoder 200 may output, either for transmission or to a storagemedium for example, a bitstream of encoded video data that includescontext encoded syntax elements and the bypass encoded additional syntaxelements.

FIG. 16 is a flowchart illustrating an example operation of a videodecoder for decoding a current block of video data. The current blockmay include a current CU. Although described with respect to videodecoder 300 (FIGS. 1 and 8), it should be understood that other devicesmay be configured to perform an operation similar to that of FIG. 16.

Video decoder 300 determines a size of a transform unit (610). Thetransform unit may include N coefficient groups, and each coefficientgroup may include M coefficients. M and N are integer values. In oneexample, the transform unit may be a luma transform unit, and M mayequal 16. In another example, the transform unit may be a chromatransform unit, and M may equal 4.

Video decoder 300 determines a regular bin count threshold for thetransform unit based on the size of the transform unit (612). Todetermine the regular bin count threshold for the transform unit basedon the size of the transform unit, video decoder 300 may, for example,set the regular bin count threshold for the transform unit equal toN*28. To determine the regular bin count threshold for the transformunit based on the size of the transform unit, video decoder 300 may, forexample, set the regular bin count threshold for the transform unitequal to 1.75 multiplied by a total number of coefficients in thetransform unit.

In one example, to determine the size of the transform unit, videodecoder 300 may determine a size of a non-zeroed out region of thetransform unit, and to determine the regular bin count threshold for thetransform unit based on the size of the transform unit, video decoder300 may set the regular bin count threshold for the transform unit equalto 28 multiplied by a number of coefficient groups in the non-zeroed outregion of the transform unit. In another example, to determine the sizeof the transform unit, video decoder 300 may determine a size of anon-zeroed out region of the transform unit, and to determine theregular bin count threshold for the transform unit based on the size ofthe transform unit, video decoder 300 may set the regular bin countthreshold for the transform unit equal to 1.75 multiplied by a totalnumber of coefficients in the non-zeroed out region of the transformunit. In an example where the transform unit is a chroma transform unit,to determine the regular bin count threshold for the transform unitbased on the size of the transform unit, video decoder 300 may set theregular bin count threshold for the transform unit equal to N*7.

Video decoder 300 context decodes syntax elements for the transform unituntil the first number of regular coded bins is reached (614). Thecontext decoded syntax elements may, for example, include one or moresignificance flags (e.g., SIG as described above), one or more paritylevel flags (e.g., Par as described above), and one or more first flags(e.g., Gt1 and/or Gt2 as described above).

In response to reaching the first number of regular coded bins, videodecoder 300 bypass decodes additional syntax elements of the transformunit (616). To bypass decode the additional syntax elements, videodecoder 300 may decode the additional syntax elements using one or bothof Golomb-Rice coding or Exponential-Golomb coding.

Video decoder 300 determines values for a first set of coefficients ofthe transform unit based on the context decoded syntax elements (618).Video decoder 300 may, for example, determine the first set ofcoefficients using SIG, Par, Gt1, and Gt2 values, as well as remLevelvalues as described above. Referring back to FIG. 5, the first set ofcoefficients may, for example, include coefficients 0 through K.

Video decoder 300 determines values for a second set of coefficients ofthe transform unit based on the additional syntax elements (620). Videodecoder 300 may, for example, determine the second set of coefficientsusing only remLevel values determined in the manner described above.Referring back to FIG. 5, the second set of coefficients may, forexample, include coefficients K+1 through N.

Video decoder 300 may also determine a decoded transform block based onthe values for the first set of coefficients and the values for thesecond set of coefficients; add the decoded transform block to aprediction block to determine a reconstructed block; perform one or morefiltering operations on the reconstructed block to determine a decodedblock of video data; and output a decoded picture of video data thatincludes the decoded block of video data.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can include one or more of RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage, or other magnetic storagedevices, flash memory, or any other medium that can be used to storedesired program code in the form of instructions or data structures andthat can be accessed by a computer. Also, any connection is properlytermed a computer-readable medium. For example, if instructions aretransmitted from a website, server, or other remote source using acoaxial cable, fiber optic cable, twisted pair, digital subscriber line(DSL), or wireless technologies such as infrared, radio, and microwave,then the coaxial cable, fiber optic cable, twisted pair, DSL, orwireless technologies such as infrared, radio, and microwave areincluded in the definition of medium. It should be understood, however,that computer-readable storage media and data storage media do notinclude connections, carrier waves, signals, or other transitory media,but are instead directed to non-transitory, tangible storage media. Diskand disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and Blu-raydisc, where disks usually reproduce data magnetically, while discsreproduce data optically with lasers. Combinations of the above shouldalso be included within the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore DSPs, general purpose microprocessors, ASICs, FPGAs, or otherequivalent integrated or discrete logic circuitry. Accordingly, the term“processor,” as used herein may refer to any of the foregoing structureor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method for decoding video data, the methodcomprising: determining a size of a non-zeroed out region of a transformunit; defining a regular bin count threshold for the transform unit as anumber of bins equal to 1.75 multiplied by a total number ofcoefficients in the non-zeroed out region of the transform unit; contextdecoding syntax elements in groups of 4 for the transform unit until theregular bin count threshold is reached, wherein: the context decodedsyntax elements comprise one or more significance flags and one or moreparity level flags, each of the one or more significance flags indicateif an absolute level for a corresponding coefficient is equal to zero,each of the one or more parity level flags indicates if a correspondingcoefficient has an absolute level that is even or odd, in response tothe context decoded syntax elements reaching the regular bin countthreshold, bypass decoding additional syntax elements of the transformunit; determining values for a first set of coefficients of thetransform unit based on the context decoded syntax elements; anddetermining values for a second set of coefficients of the transformunit based on the additional syntax elements.
 2. The method of claim 1,wherein the transform unit comprises N coefficient groups and eachcoefficient group comprises M coefficients, wherein M and N are integervalues, wherein the transform unit comprises a luma transform unit and Mequals
 16. 3. The method of claim 1, wherein bypass decoding theadditional syntax elements comprises decoding the additional syntaxelements using one or both of Golomb-Rice coding or Exponential-Golombcoding.
 4. The method of claim 1, further comprising: determining adecoded transform block based on the values for the first set ofcoefficients and the values for the second set of coefficients; addingthe decoded transform block to a prediction block to determine areconstructed block; performing one or more filtering operations on thereconstructed block to determine a decoded block of video data; andoutputting a decoded picture of video data that includes the decodedblock of video data.
 5. A method for encoding video data, the methodcomprising: determining a size of a non-zeroed out region of a transformunit; defining a regular bin count threshold for the transform unit as anumber of bins equal to 1.75 multiplied by a total number ofcoefficients in the non-zeroed out region of the transform unit; contextencoding syntax elements in groups of 4 for the transform unit until theregular bin count threshold is reached, wherein: the context encodedsyntax elements comprises one or more significance flags and one or moreparity level flags, each of the one or more significance flags indicateif an absolute level for a corresponding coefficient is equal to zero,each of the one or more parity level flags indicates if a correspondingcoefficient has an absolute level that is even or odd, and the contextencoded syntax elements represent values for a first set of coefficientsof the transform unit; and in response to the context encoded syntaxelements reaching the regular bin count threshold, bypass encodingadditional syntax elements for the transform unit, wherein theadditional syntax elements represent values for a second set ofcoefficients of the transform unit.
 6. The method of claim 5, whereinthe transform unit comprises N coefficient groups and each coefficientgroup comprises M coefficients, wherein M and N are integer values,wherein the transform unit comprises a luma transform unit and M equals16.
 7. The method of claim 5, wherein bypass encoding the additionalsyntax elements comprises encoding the additional syntax elements usingone or both of Golomb-Rice coding or Exponential-Golomb coding.
 8. Adevice for decoding video data, the device comprising: a memoryconfigured to store video data; and one or more processors implementedin circuitry and configured to: determine a size of a non-zeroed outregion of a transform unit; define a regular bin count threshold for thetransform unit as a number of bins equal to 1.75 multiplied by a totalnumber of coefficients in the non-zeroed out region of the transformunit; context decode syntax elements in groups of 4 for the transformunit until the regular bin count threshold is reached, wherein: thecontext decoded syntax elements comprise one or more significance flagsand one or more parity level flags, each of the one or more significanceflags indicate if an absolute level for a corresponding coefficient isequal to zero, and each of the one or more parity level flags indicatesif a corresponding coefficient has an absolute level that is even orodd, in response to the context decoded syntax elements reaching theregular bin count threshold, bypass decode additional syntax elements ofthe transform unit; determine values for a first set of coefficients ofthe transform unit based on the context decoded syntax elements; anddetermine values for a second set of coefficients of the transform unitbased on the additional syntax elements.
 9. The device of claim 8,wherein the transform unit comprises N coefficient groups and eachcoefficient group comprises M coefficients, wherein M and N are integervalues, wherein the transform unit comprises a luma transform unit and Mequals
 16. 10. The device of claim 8, wherein to bypass decode theadditional syntax elements, the one or more processors are configured todecode the additional syntax elements using one or both of Golomb-Ricecoding or Exponential-Golomb coding.
 11. The device of claim 8, whereinthe one or more processors are further configured to: determine adecoded transform block based on the values for the first set ofcoefficients and the values for the second set of coefficients; add thedecoded transform block to a prediction block to determine areconstructed block; perform one or more filtering operations on thereconstructed block to determine a decoded block of video data; andoutput a decoded picture of video data that includes the decoded blockof video data.
 12. A device for encoding video data, the devicecomprising: a memory configured to store video data; and one or moreprocessors implemented in circuitry and configured to: determine a sizeof a non-zeroed out region of a transform unit; define a regular bincount threshold for the transform unit as a number of bins equal to 1.75multiplied by a total number of coefficients in the non-zeroed outregion of the transform unit; context encode syntax elements in groupsof 4 for the transform unit until the regular bin count threshold isreached, wherein: the context encoded syntax elements comprises one ormore significance flags and one or more parity level flags, each of theone or more significance flags indicate if an absolute level for acorresponding coefficient is equal to zero, each of the one or moreparity level flags indicates if a corresponding coefficient has anabsolute level that is even or odd, and the context encoded syntaxelements represent values for a first set of coefficients of thetransform unit; and in response to the context encoded syntax elementsreaching the regular bin count threshold, bypass encode additionalsyntax elements for the transform unit, wherein the additional syntaxelements represent values for a second set of coefficients of thetransform unit.
 13. The device of claim 12, wherein the transform unitcomprises N coefficient groups and each coefficient group comprises Mcoefficients, wherein M and N are integer values, wherein the transformunit comprises a luma transform unit and M equals
 16. 14. The device ofclaim 12, wherein to bypass encode the additional syntax elements, theone or more processors are configured to encode the additional syntaxelements using one or both of Golomb-Rice coding or Exponential-Golombcoding.